DATESTYLE and 0000-00-00

Greetings,

I have my DATESTYLE set to ISO MDY.

When I try to create a table with a default date of 0000-00-00, psql says 
that this is an invalid date.  Why, and can (or how can I) get it to 
accept 0000-00-00 as a valid date?

Thanks,
-Josh